1. Field of the Invention
The invention relates to an image pick-up apparatus which includes
a plurality of image sensors for picking up sub-images, and PA1 a correction system for deriving corrected brightness values from brightness values of the sub-images and a correction signal. PA1 the correction system includes a light source for illuminating at least one of the image sensors in order to generate one or more measurement signals from the illuminated image sensor (sensors), and PA1 the correction system is arranged to derive the correction signal from the one or more measurement signals.
The invention also relates to an X-ray examination apparatus.
2. Description of the Related Art
An image pick-up apparatus and an X-ray examination apparatus are known from the international application WO 97/00573, which is incorporated herein by reference in its entirety and for all purposes.
The known X-ray examination apparatus includes an X-ray source and an X-ray image intensifier whereto the image pick-up apparatus is optically coupled. An X-ray image is formed of an object, for example a patient to be radiologically examined who is arranged between the X-ray source and the X-ray image intensifier, by irradiating the object by means of an X-ray beam emitted by the X-ray source. The X-ray image is formed on an entrance screen of the X-ray image intensifier and converted into an optical image on an exit window of the X-ray image intensifier. The optical image is picked up by the image pick-up apparatus and converted into an electronic image signal. The light emanating from the exit screen is split into two sub-images by means of an image splitter which includes a partly transparent mirror, said sub-images being picked up by respective image sensors.
The known image pick-up apparatus includes two solid state image sensors, each of which comprises a plurality of photosensitive elements. The known image pick-up apparatus also includes a combination unit for deriving a composite image signal from the sub-image signals. The image sensors are arranged in such a manner that pixels of the optical image which are picked up by one image sensor will be situated in the intermediate spaces between the photosensitive sensor elements of the other image sensor. Each image sensor picks up a respective sub-image and applies a respective sub-image signal to the combination unit which forms a composite image signal for a composite image from said sub-image signals. From the sub-images the image sensors derive electronic sub-image signals having signal levels which represent brightness values in the sub-images. The composite image is formed by way of the corrected brightness values of the sub-images. The composite image is composed, for example of image lines of the sub-images which have been picked up by the individual sensors, for example in such a manner that image lines from one image sensor constitute the odd image lines of the composite image and image lines from the other image sensor constitute the even image lines in the composite image. In the direction transversely of the image lines the composite image has a spatial resolution which has approximately been doubled in comparison with the sub-images picked up by the individual sensors.
Differences are liable to occur between brightness values of image lines from different image sensors, for example due to differences between the sensitivities of individual image sensors. The sensitivity of such an image sensor is represented by the ratio of the intensity of the light incident on the image sensor to the signal level of the image signal produced by the light in the image sensor. Such differences between the sensitivities of individual image sensors may cause disturbances in the composite image if no correction is made for differences between the sensitivities of the individual image sensors. The composite image may exhibit, for example a streaky pattern which is not at all related to the image information in the optical image but is caused by unequal sensitivities of the individual image sensors. The known image pick-up apparatus includes a correction system for correcting the brightness values of the sub-images for such differences, thus avoiding the occurrence of disturbances in the composite image which are caused by differences between the sensitivities of individual image sensors. The accuracy of the correction of the brightness values of the sub-images is dependent on the optical image. This is because the correction system of the known image pick-up apparatus derives the correction signal from a bright part of the optical image. Accurate correction can be achieved only when an optical image is available which contains a rather large part of considerable brightness.